MIMO, as a well known important technology in the current wireless system, creates multiple spacial channels through setting multiple transmit and receive antennas at the transmit end and the receive end, such that the capacity of uplink and downlink capacities can be improved and transmission of wireless signals can be further improved. For the MIMO technology, it is required to obtain channel state information at the transmit end so as to realize spatial multiplexing. In the case of FDD (frequency-division multiplexing), a base station (eNB) must rely on a feedback from a user equipment (UE) side to obtain such kind of channel state information. It has been already aware that the accuracy of quantization and feedback of the channel from the user terminal side should be improved to enhance the LTE-A downlink MIMO technology.
The hierarchical feedback as described in the literature “A Closed-Loop Multiple-Input-Multiple-Output Scheme For Wireless Communication Based On Hierarchical Feedback” submitted by F. Boccardi, H. Huang, and A. Alexiou on May 29, 2007 is a promising candidate scheme. The basic idea of the hierarchical feedback scheme is that if the channel changes sufficiently slowly, then the mobile channel direction indication (CDI) feedback could be aggregated over multiple feedback intervals, so as to index a larger codebook using aggregated bits. Generally, a larger codebook means more accurately describing the MIMO channel state, which is undoubtedly helpful to perform more optimized precoding at a base station side to thereby enhance system performance such as resource utilization and data throughput.
Based on such hierarchical feedback scheme, first, it is required to generate a hierarchical codebook that may accurately depict the channels experienced by the user terminal; second, it is required to use a PMI feedback signaling based on the hierarchical codebook to perform channel state information (such as CDI) transmission.
A hierarchical codebook generation method has been proposed in the paper “On transceiver design and channel quantization for downlink multiuser MIMO systems with limited feedback” (IEEE Journal on selected Areas in communications, Vol. 26, no. 8, October 2008”) by M. Trivellato, F. Boccardi and H. Huang, and all content of which is incorporated here by reference. The binary process generated by the hierarchical codebook may be represented by a binary tree of levels, the codewords on the ith level of the codebook having 2 elements. In the generation procedure, the codewords are arranged into the structure of the hierarchical codeword tree. It would be appreciated that for the above hierarchical feedback, through aggregating feedback bits over multiple intervals, the codewords indexed by feedback over a given interval are usually associated nodes of the codewords indexed by the feedback over the preceding time interval.
Based on this reference literature, given the transmit antenna number M and the maximum bit numbers of the hierarchical codebook tree Bmax, the codewords in the codebook may be designed into the hierarchical tree structure of FIG. 1 through the following steps:    1) At the level L=log2(M), the total number of codewords M in this level is 2L, including M orthogonal unit norm vectors derived from DFT (Discrete Fourier Transformation) codebook, where M is the transmit antenna number;    2) Compute a partition region relative to each codeword and obtain L subsets of training samples;    3) For level L+1, L+2, . . . Bmax, compute an optimum codebook with 2 vectors for each subset using e.g. LBG algorithm and split each subset into 2 subsets.
FIG. 1 shows a hierarchical tree (binary tree) structure representing codeword arrangement of a hierarchical codebook. Each dotted line in the figure represents one level, and a dot at the intersection of a solid line and a dotted line represents a codeword on that level. It may be seen that the first level has two codewords, the second level has four codewords, the third level has eight codewords, and the Lth level has 2L codewords, and so forth. From the binary tree in the figure, hierarchical partition of the hierarchical codebook space can be seen intuitively.
However, the brand-new hierarchical codebook as generated above is hardly accepted by a standard such as LTE 10. On one hand, such a hierarchical codebook doesn't inherit good codebook properties like constant modular, unitary, nested structure, finite alphabet, etc., which are highly desired from the perspective of many companies (see 3GPP Tdoc R1-101674, “Views on Codebook Design for Downlink 8Tx MIMO”, NTT Docomo). On the other hand, LTE R10 will be finished at the end of 2010, and the remained timeline is too tight to have sufficient efforts to define such a new codebook (about 2 years were taken to define the LTE R8 codebook).
In this case, it is desirable to propose a new channel state information feedback solution, comprising an improved hierarchical codebook generating method, such that the generated codebook inherits not only the existing good codebook properties but also the merits of hierarchical codebook to improve channel feedback accuracy; correspondingly, it further comprises an improved feedback signaling mechanism to provide subsequent application of the codebook to obtain a hierarchical feedback solution supported by the LTE R10.